Conventional offset-printing inks contain petroleum solvent, a solvent derived from petroleum distillates. However, problems have been pointed out with petroleum solvents, such as the exhaustion of the oil resources and harm to the work environments in which they are used.
The offset ink industry is accelerating the reduction of VOC (volatile organic compounds) emissions to make inks friendlier to the environment, less damaging to the air and less harmful to the human body for example. The mainstream is environmentally friendly low-VOC ink products, which contain no organic solvents such as toluene and MEK (methyl ethyl ketone). There is a need for fine-grained pigments that disperse well even in environmentally friendly ink systems, which are free from the organic solvents that have conventionally been used as dispersants for fine-grained pigments, such as toluene and MEK.
A pigment for use as a coloring agent needs to have high color strength, high chroma, and high dispersion stability. To achieve the high color strength and high chroma, pigments having small grain diameters with a narrow grain size distribution are desirable. The dispersion stability is achieved through the use of pigment surface modifiers and treatment with a derivative.
In a process described in the literature, two quinacridone compounds are ground in the presence of an effective solubilizing amount of an alcohol and a base, and the resulting solid solution is isolated. With this process, however, dispersion stability is difficult to achieve. The color strength and chroma of the quinacridone pigment produced are not high either (PTL 1).
In a process set forth in the literature for the production of a quinacridone, a quinacridone derivative, and a quinacridone solid solution, oxidation is performed in the presence of a base, dimethylsulfoxide, water, and a catalyst. This process, too, fails to achieve sufficient color strength (PTL 2).
A proposed quinacridone solid-solution pigment contains an unsubstituted quinacridone and 4,11-dichloroquinacridone and exhibits characteristic peaks in X-ray diffraction. This pigment, likewise, does not meet requirements for the chroma and color strength characteristics (PTL 3).